Illusion is an recurring element in the Sonic the Hedgehog series. They serve as power-up items and attacks for the player which can deceive the vision of the opponent(s) in gameplay.
Sonic Rivals series
In Sonic Rivals and Sonic Rivals 2, Illusion is one of the elemental Power-Ups that is useable to all playable characters. Should the player get affected by it, their screen will turn grayscale and be heavily distorted, making it harder to see the Act in gameplay. This effect is only temporarily though.
When used offensively, the user's Rival will be instantly affected by Illusion and when used defensively, the user will drop a green spiralling orb of energy that will trigger the effect if their Rival runs into them.
Sonic Forces: Speed Battle
In Sonic Forces: Speed Battle, Illusion is one of the elemental effects which the Illusion Boost and Illusion Mine inflict to hinder their opponents. When hit, the player's screen flips horizontally temporarily, reversing the player's controls when changing lanes. An icon will also appear near the top-center of the screen to indicate the reversed controls. These effects are only temporarily though. Illusion is notably one of the only effects that can affect players even if they are protected by the Shield or Invincibility.